Networks serve the purpose of coupling data between many remotely spaced computing devices, such as computers, file servers, printers etc., so that valuable computing resources can be shared amongst the various devices.
A commonly known technique for transmitting data across a network is to break the data file into smaller frames, each of which is individually modulated onto a carrier and transmitted on a network medium to a remote destination. At the destination, the carrier is demodulated to recover the data and the frames are sequenced to recover the data file.
Each frame includes a portion of the data file along with overhead data for routing the frame to the destination device. When such technique is used in a network, the network is commonly known as a frame-switched network or packet-switched because each frame, or packet, can be routed to a different destination across a multiple access topology.
In the absence of any distortion of the carrier signal across the network medium, the received carrier would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated without error using known mixing techniques, the digital data could be recovered using known sampling algorithms, and the data file can be readily recovered by simply re-sequencing the frames.
However, the network topology tends to distort the carrier signal. In a multiple access cable network, the distortions are typically due to reflections of the transmitted carrier caused by numerous branch connections and different lengths of such branches. Such problems are even more apparent in a network which uses home telephone wiring cables as the network cable medium because the numerous branches and connections are typically designed for transmission of plain old telephone system POTS signals in the 0.3–3.4 kilohertz frequency range and are not designed for transmission of high speed data signals.
Such distortion of frequency, amplitude, and phase of the carrier signal degrades network performance and tends to impede the design of higher data rate networks and challenges designers to continually improve modulation techniques and data recovery techniques to improve data rates. For example, under the Home Phoneline Networking Association (HPNA) 1.0 standard, a 1 Mbit data rate is achieved using pulse position modulation (PPM) of a carrier, while the more recent HPNA 2.0 standard achieves a 10 Mbit data rate using a complex modulation scheme utilizing a frequency diverse quadrature amplitude modulation (QAM).
A problem associated with advancing standards and increasing data rates is that, as in the HPNA example, the modulation techniques are not the same. As such, backwards compatibility is not inherent in the design of the newer systems. For example, in the HPNA system, to be backwards compatible, the newer HPNA 2.0 device must be able to transmit and receive both the PPM modulated carrier compliant with the HPNA 1.0 standard and the frequency diverse QAM modulated carrier compliant with the HPNA 2.0 standard. Further, the newer HPNA 2.0 devices must be able to operate in a compatibility mode network which includes both HPNA 1.0 devices and HPNA 2.0 devices. In a compatibility mode network, HPNA 2.0 devices may not communicate directly with other HPNA 2.0 devices utilizing the QAM modulation scheme because the older HPNA 1.0 devices, without the ability to receive QAM frames, will detect the QAM frames only as noise on the network. As such, the HPNA 2.0 standard provides for use of compatibility mode frames which include a PPM modulated header and a QAM modulated body which includes gaps between QAM modulated data to emulate PPM timing gaps. The PPM modulated header is detectable by both the HPNA 1.0 device and the HPNA 2.0 devices on the compatibility mode network and operates to alert the HPNA 1.0 devices that the frame is not addressed to such device.
A problem associated with compatibility mode frames is that each of the PPM modulation scheme and the QAM modulation scheme typically require distinct physical layer circuitry for generating the modulated carrier. While both physical layer circuits typically exist in an HPNA 2.0 device because it must be able to transmit and receive both HPNA 1.0 frames and HPNA 2.0 frames, structuring the two circuits to operate together, with nearly perfect timing, to generate a single frame including both a PPM modulated header and a QAM modulated body is both difficult and expensive at best.
Therefore, based on recognized industry goals for size, cost, and power reductions, what is needed is a device and method for compatibility frame generation which does not suffer the disadvantages of known systems.